The invention relates generally to pyrimidine derivatives, and more particularly to pyrimidine derivatives which are suitable for use in nematic liquid crystal compositions to increase the N.fwdarw.I point and lower the threshold voltage.
Liquid crystal devices display information by making use of the electro-optical characteristics of liquid crystal materials. Specifically, these displays utilize the nematic liquid crystal phase. For example, the liquid crystal devices commonly include liquid crystal compounds of the twisted nematic, voltage controlled birefringence, guest-host and dynamic scattering types.
An improved twisted nematic compound, referred to as a super-twisted nematic compound has recently been developed. This super-twisted nematic compound has a twisted angle of from 180.degree. to 250.degree. compared to conventional nematic compounds which have a twist angle of 90.degree.. Further, a super-twisted voltage controlled birefringence compound having a twisted angle of 270.degree. in its stable condition has been proposed, but has not yet been developed for practical use.
Conventional liquid crystal display devices have many advantages. For example, they can be driven with low voltage and low power consumption, can be constructed in a small, flat shape and are not subject to fatigue because of the passive element structure even after use for long periods of time. These properties have allowed use of liquid crystal display devices in watches and time pieces, electronic calculators, automobile dashboards, audio mechanisms and the like. Recently an additional wide range of products, such as personal computers, word processors, pocket color televisions and other devices which had previously included CRT's.
The liquid crystal materials which are included in liquid crystal display devices must have certain properties. These are determined by the method of displaying and usage of the device. Twisted nematic liquid crystal compounds are widely used at present and their desireable properties include:
1. colorless and stable over a wide range of thermal, optical, electrical and chemical conditions; PA1 2. useful over a wide range of temperatures; PA1 3. high speed electro-optical response; PA1 4. low driving voltage; PA1 5. sharp rise time in voltage-light transmittance with a low temperature dependency; and PA1 6. wide visual range.
The low driving voltage and use over a wide range of temperatures are particularly important properties. For the twisted nematic type the following relationship exists between the threshold voltage (V.sub.th) and the dielectric anisotropy ( .epsilon.): ##EQU1## wherein K.sub.11, K.sub.12 and K.sub.33 are elastic constants for the spray, twist, and bend of the compound, respectively and k is a proportional constant.
As is evident from this formula, in order to decrease the threshold voltage a large positive value of .epsilon. is required. However, conventional liquid crystal compounds which have a large .epsilon. such as 4-alkyl benzoic acid -4-cyanophenyl ester and 4-alkyl-4'-cyanobiphenyl have a low temperature at which the nematic phase undergoes a transition into the isotropic liquid phase (N.fwdarw.I point). This renders the nematic temperature range too narrow. Compounds having a higher N.fwdarw.I point and a large .epsilon., such as 4-alkyl-4"-cyanoterphenyl, 4-(trans-4'-alkylcyclohexyl-4"-cyanobiphenyl and the like have a large elastic constant which increases the threshold voltage.
Accordingly, it is desirable to provide improved liquid crystal compositions for use in liquid crystal display devices which have high N.fwdarw.I points and a low threshold voltage.